Automobile and motorcycle exhaust gases are conventionally purified with a catalyst supported on a ceramic body able to withstand high temperatures. The preferred catalyst support structure is a honeycomb configuration which includes a multiplicity of unobstructed parallel channels sized to permit gas flow and bounded by thin ceramic walls. The channels may have any configuration and dimensions provided gases can freely pass through them without being plugged by entrained particulate material. Examples of such preferred structures include the thin-walled ceramic honeycomb structures described in U.S. Pat. No. 3,790,654 to Bagley and in U.S. Pat. No. 3,112,184 to Hollenbach.
Ceramic honeycomb catalyst supports are exposed to high temperatures resulting from contact with hot exhaust gases and from the catalytic oxidation of uncombusted hydrocarbons and carbon monoxide contained in the exhaust gas. In addition, such supports must withstand rapid temperature increases and decreases when the automobile engine is started and stopped. Such operating conditions require the ceramic honeycomb catalyst support to have a high thermal shock resistance, a property generally inversely proportional to the coefficient of thermal expansion.
Ceramic supports for catalytic converters are typically formed from brittle, fireproof materials such as aluminum oxide, silicon oxide, magnesium oxide, zirconium silicate, cordierite, or silicon carbide. The typical honeycomb configuration of supports made from these ceramic materials enables even very small mechanical stresses to cause cracking or crushing. In view of their brittleness, a great effort has been expended to develop catalytic converter housings, or cans, for such supports.
For example, U.S. Pat. No. 4,863,700 to Ten Eyck discloses a catalytic converter system wherein a frangible ceramic monolith catalyst is resiliently mounted in a metallic housing by an insulating layer of ceramic fibers wrapped around the monolith, and a layer of intumescent material disposed between the metal housing and the ceramic fiber layer.
In many applications, particularly those involving small engines, there is little room for mounting catalytic converters. Examples of such applications include chainsaws, lawnmowers, and motorcycles, particularly motorcycles having less than a 250 cc engine. One solution to this problem is to mount the catalytic converter within existing exhaust system components rather than providing an additional catalytic converter housing. One such location is inside a hot gas chamber. Conventional hot gas chambers include expansion chambers and mufflers, as will be discussed below, in which an exhaust pipe empties into a chamber housing with a larger cross-sectional area than the exhaust pipe. The larger cross-sectional area than the exhaust gases to expand and provides an area in which noise may be muffled.
When the catalytic converter is mounted within an expansion chamber or muffler of a motorcycle, the back pressure is substantially higher than that in an automotive converter. Due to the high back pressure, as well as the vibrational acceleration at high engine speeds, ceramic catalysts for such applications must be capable of withstanding relatively large axial forces to prevent movement in the can housing.
In addition, the high levels of carbon monoxide and hydrocarbons emitted by engine exhausts, particularly those of two-stroke motorcycles, such catalytic converters to endure extreme exotherms during pollutant oxidation. In motorcycles, this commonly results in a temperature increase from an average of 400.degree. C. at the inlet of the converter to 900.degree. C. at the outlet of the converter. Encapsulation within an insulated hot gas chamber such as a muffler prevents such converters from efficiently dissipating heat to the atmosphere. Furthermore, in such applications, the hot exhaust gas not only flows through the catalytic converters, but also around its housing. Consequently, in such applications the temperature of the catalytic converter housing assembly (i.e., the housing which maintains the converter in its correct position inside the hot gas chamber) commonly approaches 900.degree. C. Many of the conventional intumescent insulating materials used to house catalytic converters contain vermiculite, which loses its chemically-bound water when exposed to such high temperatures. The loss of chemically-bound water damages the intumescent character of the mater so that it does not provide adequate mounting pressure to retain the ceramic catalyst support. This jeopardizes the ability of the ceramic catalyst to withstand axial and other forces resulting from exhaust gas flow and vehicle vibration.
U.S. Pat. No. 4,925,634 to Yokokoji, et al. discloses a catalytic converter for use with a car, motorcycle or the like. In one embodiment, the catalytic converter housing is surrounded by an insulating dead air space to prevent thermal damage to the outer tube of the muffler.
U.S. Pat. No. 3,597,165 to Keith et al. discloses a catalytic exhaust purification device including a housing within which is mounted four ceramic catalyst supports, each catalyst being surrounded by a single layer of insulation material. The exhaust gases are directed in a straight line through the catalytic converter, which is not an efficient design for absorbing sound.
U.S. Pat. No. 4,710,487 to Koch discloses a diesel exhaust gas catalyst in which exhaust gas in routed first around and then through a ceramic catalyst. The ceramic catalyst is embedded in a single layer of an embedding material and enclosed in a catalyst holder. No mentioned is made of wrapping the catalyst in an insulating mat.
The need thus continues for a catalytic converter which will remain securely mounted inside a hot gas chamber, even at operating temperatures exceeding 800.degree. C.